BAGHDAD, Iraq (CNN) -- One group of Blackwater USA contractors was involved in two separate shootings on September 16, according to a senior Iraqi National Police official who contributed to a report detailing the second shooting.

The official also told CNN that a second group of Blackwater contractors on their way to assist the first group later engaged in an hour-long standoff with Iraqi police.

The first, widely publicized shooting occurred at Nisoor Square in Baghdad. Accounts differ, but at least 11 Iraqi civilians were killed during a firefight that involved a Blackwater convoy.

The second shooting occurred minutes later about 550 yards (500 meters) from Nisoor Square, the Iraqi National Police official told CNN Monday. The official asked for anonymity because of the sensitivity of the issue.

Blackwater did not immediately respond to a request for comment, but a senior State Department official said there were not two incidents on September 16.

Blackwater has said its employees responded properly that day to an insurgent attack on a convoy, and a State Department "spot report" underscores that and doesn't mention civilian casualties.

The second incident allegedly took place after a Blackwater convoy ran into a traffic jam caused by civilians trying to flee Nisoor Square, the official said.

When the Blackwater convoy tried to drive against the traffic flow, he said, an Iraqi police vehicle -- its officers unaware of the Nisoor shooting -- tried to open the road by stopping civilian vehicles from driving toward the convoy.

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But the official said Iraqi police report found that the contractors opened fire at the police and civilian vehicles, wounding at least three civilians.

The U.S. Embassy convoy that Blackwater contractors were guarding came under fire from insurgents at Nisoor, according to the company. However, Iraqi authorities said the shooting was unprovoked.

The senior Iraqi police official told CNN that helicopters apparently belonging to Blackwater participated in the first incident.

"Blackwater's little blue choppers were flying overhead, and there is evidence and witness testimony that they were firing, too," he said.

The evidence left at Nisoor Square included large holes in car roofs that could only come through the use of air firepower, he said. Blackwater has denied any aircraft opened fire during the incident.

The Iraqi National Police official said he arrived at Nisoor Square moments after the Blackwater convoy had left the second shooting. Describing "chaos" at Nisoor Square, he said he saw civilians killed, panicked witnesses and bullet casings, spent smoke grenades and grenade casings strewn on the ground.

Meanwhile, the official said, his team of police officers stopped a second Blackwater team that had come from Baghdad's International Zone to back up the first team, which had become mired in traffic.

Believing the second team was actually the one involved in the shooting, the officers surrounded the contractors and engaged in a standoff with them for about an hour, the official said.

He said he asked the contractors to "hold fire or we will fire; and if we do, you will fire more, so please, I ask you to point your weapons upwards."

No shots were fired and the Blackwater contractors returned to the Green Zone, he said.

The initial State Department report states that upon finding their escape route blocked by an Iraqi quick-reaction force that pointed heavy machine guns at one vehicle in the convoy, Blackwater called for backup.

A U.S. Army force, backed by air cover, arrived about half an hour later to escort the Blackwater convoy back to the Green Zone, the report states.

The State Department, assisted by the FBI, is conducting an investigation of the incident, and the House Oversight and Government Reform Committee is scheduled to hold a hearing on the matter Tuesday.E-mail to a friend